OFDM communication systems are becoming widely applied in wireless communication systems due to the high rate transmission capability with high bandwidth efficiency and robustness with regard to multi-path fading and delay. A fundamental underlying principle of OFDM systems is the division of available frequency spectrum into several sub carriers. To obtain a high spectral efficiency, the frequency responses of the subcarriers are overlapping and orthogonal. This orthogonality can be completely maintained with a small price in a loss in signal to noise ratio, even though the signal passes through a time dispersive fading channel, by introducing a cyclic prefix.
A block diagram of a baseband OFDM system is shown in FIG. 1. Binary information is firstly grouped, coded and mapped according to the modulation in a signal mapper 10. After a guard band is inserted by guard band insertion block 12, an N-point inverse discrete-time Fourier transform (IDFT) block 14 transforms the data sequence into the time domain. Following the IDFT block 14, a cyclic prefix is inserted by cyclic prefix insertion block 16 to avoid intersymbol and intercarrier interference. A D/A converter 18 transforms the digitized signal into an analogue form for transmission across a channel. The channel 20 is modeled as an impulse response g(t) followed by the complex additive white Gaussian noise n(t).
At a receiver 22, after passing through an analogue-to-digital (A/D) converter 24 and removing the cyclic prefix at cyclic prefix deletion block 26, a Discrete Time Fourier Transform (DFT) block 28 is used to transform the data back into the frequency domain. After guard band deletion by the guard band deletion block 30, the binary information data is finally obtained back after demodulation and channel decoding by the channel decoding and demodulation block 32.
Such an OFDM system is equivalent to a transmission of data over a set of parallel channels. As a result, the fading channel of an OFDM system can be viewed as a 2D lattice in a time-frequency plane, which is sampled at pilot positions. The channel characteristics between pilots are estimated by interpolation.
One exemplary channel estimation scheme used in OFDM systems is depicted in FIG. 2. In this example, OFDM channel estimation symbols are periodically transmitted, and all subcarriers are used as pilots. The receiver 22 acts to estimate the channel conditions (specified by channel coordination matrix) given the pilot signals (specified by pilot signal matrix X) and received signals (specified by received signal matrix Y). The receiver 22 uses the estimated channel conditions to decode the received data inside the block until the next pilot symbol arrives. The estimation can be based on least square (LS), minimum mean-square error (MMSE), and modified MMSE.
An LS estimator minimizes the parameter ( Y−X H)H( Y−X H), where (•)H means the conjugate transpose operation. It is shown that the LS estimator of the matrix H is given by:ĤLS=X−1 Y=[(Xk/Yk)]T(k=0, . . . , N−1)
However, these estimates are frequently found to be not accurate enough to decode transmitted information, particularly when a received signal strength is poor and/or during highly varying channel conditions. There currently exists a need to provide an improved method of channel estimation for a control channel in an OFDM system which takes into account one or more of the channel conditions in time and frequency, and the quality of the received signal to improve performance, whilst keeping the complexity and processing delays low. There also exists a need to provide a method of channel estimation for a control channel in an OFDM system that ameliorates or overcomes one or more disadvantages or inconveniences of known channel estimation methods.